2?4 DISCOVERY REPORTS 



arrive on the whaling grounds at this time of year is composed mainly of males, at least 

 in the South Georgia district. As in Blue whales, however, the females are in excess of 

 males towards the end of the season. 



Apart from these features it cannot be said that the data indicate any important dif- 

 ferences in the sex ratio in different months in the Antarctic as a whole. Some irregular 

 fluctuations take place which may be caused by more than one factor and which cannot 

 be explained from the available material. 



MONTHLY AVERAGE LENGTHS AND PERCENTAGE OF IMMATURE WHALES 

 Both at South Georgia and on the pelagic whaling grounds the proportion of small 

 immature whales in the catches increases in the later part of the summer season. 

 Mackintosh and Wheeler (1929, pp. 456-60) found evidence of this in the earlier data 

 collected at South Georgia, and from the numbers of corpora lutea in the ovaries inferred 

 (pp. 460-61) that, of the adult whales, those taken early in the season were mostly older 

 than those taken later. Wheeler (1934, p. 235 and fig. 1) showed that, of the Fin whales 

 examined at South Georgia, ' the majority of physically mature females appeared in 

 December, the majority of sexually mature not physically mature females in January and 

 the peak of the influx of immatures was in February'. This increased proportion of 

 young whales in late summer is less obvious in the Antarctic pelagic catches, but Hjort, 

 Lie and Ruud (1935, p. 21) give the monthly percentages of mature and immature Blue 

 whales taken in area III in the season 1934-5, and show that there is a distinct increase 

 in the ratio of immatures from December to March. Ottestad (1938) gives preliminary 

 length-frequency curves for each month for Blue whales in 1932-3 in areas III and IV 

 and for Fin whales in 1934-5 in areas II-IV. These curves also show an increasing pro- 

 portion of small whales in the later months, but the tendency is more noticeable in 

 area III than in areas II and IV. He concludes that this is due to the later arrival of 

 small whales and probably also to the departure of some of the larger ones. 



In Table 26 the material from the ' Southern Empress ' is analysed in respect of the 

 monthly average length and percentage immature, and corresponding figures are given 

 for the South Georgia data, though Humpbacks were too few to be included in the 

 latter. The same figures are graphed in Fig. 9. The percentage immature is calculated 

 from the revised mean lengths at maturity (see p. 219). If the original lengths were used 

 the percentages of immature Blue females and Fin males and females would mostly be 

 about 2-5 % higher. It will be recalled that the mean length at which Humpbacks be- 

 come mature was estimated on a rather small amount of data, but even if this length 

 needed some correction the rate of change from month to month in the ratio of im- 

 mature whales would hardly be affected. The table shows that in each species and sex the 

 tendency is towards a fall in the average length and an increase in the percentage im- 

 mature as the season advances. This change is more marked at South Georgia than in 

 the catches of the ' Southern Empress ' which were in area IV, and it seems more evident 

 in Blue than in Fin whales. There is very little change in the average lengths of Hump- 

 backs, but there is a distinct rise in the percentage immature. The curves for South 



